A light emitting element formed using a light emitting material has characteristics such as being thin, lightweight, having high response speed and low voltage driving. Therefore, there are high expectations for its application in the next-generation flat panel displays. Moreover, it is said that a light emitting device in that light emitting elements are arranged in a matrix form has advantages of a wider viewing angle and better visibility than a conventional liquid crystal display device.
A basic structure of a light emitting element is a structure in which a layer containing an organic compound with a light emitting property (a light emitting layer) is interposed between a pair of electrodes. By applying voltage to this light emitting element, electrons and holes are respectively transported to the light emitting layer from the pair of electrodes so that a current flows through the light emitting element. Then, by recombining these carriers (i.e., the electrons and the holes), the organic compound with the light emitting property is excited. Upon returning to a ground state from the excited state, light is emitted.
As the excited state caused by an organic compound, there are a singlet excited state and a triplet excited state. Light emission generated in a singlet excited state is referred to as fluorescence and light emission generated in a triplet excited state is referred to as phosphorescence.
Such a light emitting element is generally formed to have a thickness of 1 μm or less, for example, 0.1 μm. This light emitting element has an great advantage of being thin and lightweight. Also, since the time it takes from the injection of carriers to light emission is about 1μ second or less, the light emitting element also has an advantage of having high response speed. In addition, sufficient light emission can be obtained by a direct-current voltage of only about several to several tens of volts, so the light emitting element requires relatively low power consumption. Because of these advantages, the light emitting element has been attracting attention as a next-generation flat panel display element.
Moreover, a light emitting element is formed to have a film form. Therefore, by forming a large-area light emitting element, light emission in a sheet form can be easily obtained. It is difficult to obtain this feature in the case of using a point light source typified by a filament lamp or an LED, or a line light source typified by a fluorescent lamp. Therefore, the light emitting element has a great utility value as a sheet light source that is applicable to a lighting and the like.
However, the light emitting element has problems of short-term durability and a poor heat resistance property. These problems hinder the development of the light emitting element. Typically, as disclosed in the non patent document 1, a light emitting element is generally formed by laminating a thin organic film using an organic compound. Therefore, it is thought that the short-term durability of the organic compound and the frailty of the thin organic film cause the above described problems.
On the other hand, there is an attempt to form a light emitting element by using a layer in which an organic compound and an inorganic compound are mixed, rather than a thin organic film. For example, the patent document 1 discloses a light emitting element using a light emitting layer in which fluorescent organic molecules are dispersed in metal oxide. Further, the patent document 2 discloses a light emitting element formed by laminating layers in which an organic compound (a compound with a hole transporting property, a compound with an electron transporting property, or a compound with a light emitting property) is dispersed in silica matrix via covalent bonds. In these documents, it is reported that the durability or the heat resistance property of the light emitting elements are improved.
In such light emitting elements disclosed in the patent document 1 and 2, however, the organic compound is simply dispersed in metal oxide which has an insulating property. Therefore, the light emitting elements disclosed in the patent document 1 and 2 have a problem in which it is more difficult for the current to flow through the light emitting element (i.e., the voltage required to feed a certain amount of current is increased, that is, the resistance is increased) as compared with a conventional light emitting element.
The luminance of light emission of these light emitting elements is increased in proportion to the density of current flowing through the light emitting elements, thereby causing a problem that the voltage for obtaining a certain amount of luminance (i.e., driving voltage) is increased. This results in an increase in driving voltage along with an increase in power consumption, even if long term durability and good heat resistance property are obtained by simply dispersing the organic compound in metal oxide.
To inhibit the short circuiting of a light emitting element due to a dust and the like, it is effective to increase the thickness of the light emitting element. However, when the thickness of a light emitting element disclosed in the patent document 1 or 2 is increased, the driving voltage is further increased. That is, increasing the thickness of a light emitting element having the conventional structure is practically difficult.    [Non Patent Document 1]: C. W. Tang et al., Applied Physics Letters, vol. 51, No. 12, pp. 913-915 (1987)    [Patent Document 1]: Japanese Patent Application Laid-Open No. Hei 2-288092    [Patent Document 2]: Japanese Patent Application Laid-Open No. 2000-306669